At the present time, electronic products are used in almost every aspect of life, and the heart of these electronic products is the integrated circuit. Integrated circuits are used in everything from airplanes and televisions to wristwatches.
Integrated circuits are made in and on silicon wafers by extremely complex systems that require the coordination of hundreds or even thousands of precisely controlled processes to produce a finished semiconductor wafer. Each finished semiconductor wafer has hundreds to tens of thousands of integrated circuits, each wafer worth hundreds or thousands of dollars.
Integrated circuits are made up of hundreds to millions of individual components. One common component is the semiconductor transistor. The most common and important semiconductor technology presently used is silicon-based, and the most preferred silicon-based semiconductor device is a complementary metal oxide semiconductor (“CMOS”) transistor.
The principal elements of a CMOS transistor generally consist of a silicon substrate having shallow trench oxide isolation regions cordoning off transistor areas. The transistor areas contain polysilicon gates on silicon oxide gates, or gate oxides, over the silicon substrate. The silicon substrate on both sides of the polysilicon gate is slightly doped to become conductive. These lightly doped regions of the silicon substrate are referred to as “shallow source/drain”, which are separated by a channel region beneath the polysilicon gate. A curved silicon oxide or silicon nitride spacer, referred to as a “sidewall spacer”, on the sides of the polysilicon gate allows deposition of additional doping to form more heavily doped regions of the shallow source/drain (“S/D”), which are called “deep S/D”.
To complete the transistor, a silicon oxide dielectric layer is deposited to cover the polysilicon gate, the curved spacer, and the silicon substrate. To provide electrical connections for the transistor, openings are etched in the silicon oxide dielectric layer to the polysilicon gate and the S/D. The openings are filled with metal to form electrical contacts. To complete the integrated circuits, the contacts are connected to additional levels of wiring in additional levels of dielectric material to the outside of the dielectric material.
One improvement to the CMOS transistor uses an insulating substrate and is called silicon on insulator (“SOI”). The advantages of using an insulating substrate in CMOS and high speed field effect transistors (“FETs”) include latchup immunity, radiation hardness, reduced parasitic junction capacitance, reduced junction leakage currents, and reduced short channel effects. Many of these advantages translate to increased speed performance of the FETs.
The SOI FETs are manufactured with an insulator, such as silicon dioxide, on a semiconductor substrate, such as silicon. The entire FETs, including their source junction, channel, drain junction, gate, ohmic contacts and wiring channels, are formed on silicon islands in the insulator and are insulated from any fixed potential. This results in what is called the “floating body” problem because the potential of the body or channel regions floats and can acquire a potential which can interfere with the proper functioning of the FETs. The floating body problem causes high leakage current and parasitic bipolar action since the semiconductor substrate is floating with respect to the channel. This problem has adverse affects on threshold voltage control and circuit operation.
In order to eliminate the floating body problem, it is necessary to fully deplete the silicon island. This means making the silicon island so thin that the entire thickness of the body region is depleted of majority carriers when the FET is in the off state and both junctions are at ground. To fully deplete the silicon island and create a fully depleted silicon on insulator (“FDSOI”), it has been found that the silicon island must be extremely thin.
However, having a thin silicon island causes problems in the fabrication of FDSOI CMOS in the formation of source and drain with low parasitic series resistance. One solution is to elevate the source and drain over the thin silicon island. Elevated source and drain are formed by selective epitaxial growth (“SEG”). Unfortunately, it is difficult to uniformly grow high quality, single crystalline source and drain on the extremely thin silicon island. Furthermore, processes performed prior to SEG, such as oxidation, pre-clean, and H2 baking, can remove all or parts of the thin silicon needed for SEG.
Another key issue for fabrication of FDSOI CMOS is mechanisms to improve performance. One way to improve performance is to introduce tensile strain or compressive strain to the channel. Tensile strain along the direction of current flow increases both electron and hole mobility. On the other hand, compressive strain increases hole mobility but degrades electron mobility. Strain is introduced to the channel through trench isolation fill. However, mesa isolation, where there is no trench etch and fill, is conventionally used for FDSOI CMOS.
What is needed, therefore, is a way to uniformly grow high quality, single crystalline source and drain while introducing strain to the channel.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.